1. Field of the Invention
The present invention relates to a compression technique for compressing and coding an audio data input together with a motion picture data. Particularly, the present invention can be utilized in compressing data in a personal computer.
2. Description of Related Art
In handling a picture data and an audio data in a personal computer, a data compression/expansion technique has been used in order to reduce an amount of data. An algorithm called MPEG compression is generally well known among conventional data compression/expansion techniques. The MPEG compression is a technique for handling a large amount of data as a smaller amount of data, so that it is possible to reduce the amount of data by increasing the compression rate if a degradation of picture quality is allowable or it is possible to reduce the compression rate when a high picture quality is required. Currently, MPEG2 compression technique obtained by improving the basic MPEG compression technique is being used. With the MPEG2 compression technique, picture data is compressed at a frame rate of 6 Mbps and audio data is compressed at a sampling rate of 44.1 kHz, as the main compression level. These numerical values are based on picture quality similar to that obtained in the current television receiver and tone quality similar to that obtained by a compact disk.
In general, a picture quality depends upon a changing rate of scene and a value of bit rate. When the changing rate of scene change is low, the picture quality is not degraded substantially even if the bit rate is reduced, that is, the number of frames per unit time is reduced. However, when the changing rate of scene is high, the picture quality is degraded considerably. In other words, when the changing rate of scene is low, a large amount of data is not required so that there is no picture quality problem occurs even if the bit rate is reduced, while, when the changing rate of scene is high, the picture quality is degraded unless the amount of data is increased, resulting in a picture which is hardly watched comfortably. In view of this fact, an algorithm using a variable bit rate processing has been developed, in which a picture whose frequency of scene change is high is compressed at high bit rate, while a picture whose changing rate of scene is low is compressed at a lower bit rate.
As mentioned, the bit rate for a picture is changed correspondingly to the necessity of further reducing the amount of data and the processing thereof.
On the other hand, the amount of audio data is small compared with that of a picture so that it is usual to code the audio data at a constant sampling frequency. However, in a general purpose equipment such as a personal computer which performs almost all processing according to a software, it is desired to compress even audio data whose amount is small to some extent since a load on a central processing unit (CPU) is large.
Japanese Patent Application Laid-open No. Hei 7-303240 discloses a technique in which, in processing an audio data accompanied with a motion picture data, an audio signal is reproduced by changing a speed of the audio signal itself in reproducing a video signal at a variable speed. In order to change the audio signal speed, the Time Domain Harmonic Scaling (TDHS) technique is used, with which it is possible to reproduce the audio signal at a variable speed without changing the interval thereof. However, this technique is used to not compress an amount of audio data but reproduce a recorded audio data while changing its speed.
Japanese Patent Publication No. Sho 59-3760 discloses a technique, in which a sampling frequency for coding and a reproducing speed in decoding are selected correspondingly to a required service. In this technique, a clock rate is arbitrarily changed under control of a transfer control device correspondingly to the service to make the coding bit rate during a storage time and the decoding bit rate during a reproduction corresponding thereto variable independently. However, this technique is used to neither flexibly change the sampling frequency in one service (a series of audio data) nor make the compression rate of the audio data accompanied with a motion picture data variable.
Other well known techniques related to the compression of the audio signal as well as the picture signal and the sampling processing in compressing them are disclosed in Japanese Patent Application Nos. Sho 56-36700, Sho 64-10717, Hei 4-38767, Hei 7-154441, Hei 8-172645 and Hei 8-205092. However, these prior arts do not make the compression rate of the audio data accompanied with the motion picture data variable.